Project 3: Role of a psychiatric disease risk factor in synaptic function and gene transcription regulation ABSTRACT Genetic complexity underlying the vast majority of mental disorders has made the study of these diseases exceptionally challenging. Many risk-associated genes have been identified but the biological role is largely unknown. Dysregulated neurodevelopment with altered structural and functional connectivity is believed to underlie many neuropsychiatric disorders and ?a disease of synapses? is the major hypothesis for the biological basis of schizophrenia and other major psychiatric disorders. However, little is known about pathophysiology of synapses in patient neurons, underlying molecular and cellular mechanisms, and to what extent psychiatric disorders may share these mechanisms. Disrupted in Schizophrenia 1 (DISC1) is a gene in which mutations have been associated with increased risk for schizophrenia, bipolar disorder, and other major psychiatric disorders. A large number of animal studies have shown that DISC1 affects multiple neurodevelopmental processes, including synapse formation. Understanding synaptic dysfunction in major psychiatric disease requires direct investigation of synapse properties in human neurons derived from patients with these disorders. Reprogramming patient somatic cells enables recapitulation of normal and pathological human tissue developmental properties in defined conditions and a new way to identify the cellular processes underlying complex human diseases, which can lead to mechanism-based drug discovery. A rare mutation of a 4 base-pair frame-shift deletion at the C-terminus of DISC1 was discovered to co-segregate with major psychiatric disorders in a smaller American family (Pedigree H). The current project is built upon our initial results showing that forebrain neurons derived from iPSCs with the DISC1 mutation exhibit significant synaptic defects and RNA-seq analysis showed significant dysregulation of a large number of neuronal genes related to synaptic function and psychiatric disorders in patient neurons. Recent studies using iPSCs from idiopathic schizophrenia patients also showed similar defects in synaptic function and share multiple dysregulated genes. Project 3 will test the hypothesis that a psychiatric disorder risk gene modulates synaptic function of human neurons via biochemical and transcriptional dysregulation, a core defect that may also be present in idiopathic schizophrenia and bipolar patient-derived neurons. Aim 1 will characterize cellular phenotypes of human cortical neurons differentiated from Pedigree H and idiopathic schizophrenia patient iPSCs. Aim 2 will determine the role of mutant DISC1 in iPSC-derived astrocytes. Aim 3 will evaluate neuronal subtype specificity of mutant DISC1 effects on neuronal development, synaptic function and transcription. Each aim requires the involvement of at least one academic and one industrial partner as well as Core B at Janssen and is designed to incorporate cross-validation across labs and establish protocols that can be disseminated for further validation. Cellular phenotypes will be compared to other iPSC lines in the NCRCRG and will be further developed into miniaturized assays via Core C at SBMRI for future drug screens.